RFR: 8357828: Add a timestamp to jcmd diagnostic commands [v5]

[Ivan Bereziuk]

On Fri, 23 Jan 2026 16:26:57 GMT, Ivan Bereziuk <duke at openjdk.org> wrote:

>> `jcmd` provides great diagnostics but many commands lack a timestamp in their output.
>> Adding a timestamp to the output would add value for those debugging JVM data.
>> 
>> Some diagnostic commands already provide timestamps. For example `Thread.print` already prints one of "yyyy-MM-dd HH:mm:ss" format.
>> 
>> With this MR I propose to introduce time-stamping to all diagnostic `jcmd` commands in a form of an additional common flag "-t":
>> 
>> jcmd [pid | main-class] [-t] command... | PerfCounter.print | -f filename
>>                         ^^^^
>> 
>> * The choice for time format is ISO 8601 `yyyy-MM-dd'T'HH:mm:ss.SSSZ` (example `2026-01-21T16:58:49.518+0100`)
>> * if "-t" flag is not passed, `Thread.print` keeps printing "yyyy-MM-dd HH:mm:ss" timestamp to preserve backwards compatibility.
>
> Ivan Bereziuk has updated the pull request incrementally with one additional commit since the last revision:
> 
>   tweaked help working for -t flag

I have a side question. This might help us make the right choice.
Q: Is there a chance that we change the format of timestamp from `Thread.print` printed currently to ISO 8601?

Also. Adding `jcmd` "duration at the end" feature makes sense only if it's measured per individual command. Propagation of the argument to the target JVM is necessary because of `jcmd`'s "-f <file>" submits a chain of commands to the target JVM.
As a User I would expect "-t" to align with that and timestamp before each individual command.

Thoughts?

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PR Comment: https://git.openjdk.org/jdk/pull/27368#issuecomment-3933032044